Broadband study of the Be X-ray binary RX J0520.5-6932 during its outburst in 2024

Abstract

A new giant outburst of the Be X-ray binary RX J0520.5–6932 was detected and subsequently observed with several space-borneand ground-based instruments. Thisstudy presents a comprehensive analysis ofthe optical and X-ray data,focusing on the spectraland timing characteristics of selected X-ray observations. A joint fit of spectra from simultaneous observations performed by theX-ray telescope (XRT) on the Neil Gehrels Swift Observatory (Swift) and Nuclear Spectroscopic Telescope ARray (NuSTAR)provides broad-band parameter constraints, including a cyclotron resonant scattering feature (CRSF) at 32.2+0.8 −0.7 keV with nosignificant energy change since 2014, and a weaker Fe line. Independent spectral analyses of observations by the Lobster EyeImager for Astronomy, Einstein Probe (EP), Swift–XRT, and NuSTAR demonstrate the consistency of parameters across differentbands. Luminosity variations during the current outburst were tracked. The light curve of the Optical Gravitational LensingExperiment (OGLE) aligns with the X-ray data in both 2014 and 2024. Spin evolution over 10 yr is studied after adding FermiGamma-ray Burst Monitor data, improving the orbital parameters, with an estimated orbital period of 24.39 d, slightly differingfrom OGLE data. Despite intrinsic spin-up during outbursts, a spin-down of ∼0.04 s over 10.3 yr is suggested. For the newoutburst, the pulse profiles indicate a complicated energy-dependent shape, with decreases around 15 and 25 keV in the pulsedfraction, a first for an extragalactic source. Phase-resolved NuSTAR data indicate variations in parameters such as flux, photonindex, and CRSF energy with rotation phase.

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